Ligand binding to hemoglobin in solution will be examined in order to derive a plausible, physical-chemical description of this process and to correlate the resulls with the known structural features of the protein molecule. In particular, we have focused our attention on the binding properties of a series of straight and branched alkyl isocyanides in order to assess the importance of steric effects at the sixth coordination of the heme iron atom. In addition, the rates of O2 uptake and release by intact human red blood cells, resealed erythrocyte membrane preparations, and artificial phospholipid bilayer vesicles will be measured by dual wavelength stopped-flow kinetic techniqes. The objective of this study is to determine the relative importance of unstirred plasma layers, membrane diffusion resistance, hemoglobin concentration and modification, cell shape and size, and various diseased or anemic states on the gas transport capacities of intact erythrocytes. Any substantial impairment of ligand uptake would have serious clinical manifestations since the rate of O2 uptake by erythrocytes is already thought to limit, in part, the rate of gas exchange in the microcirculation of lung tissue. Lastly, the mechanism of heme transport and incorporation into globin will be studied in terms of the role of cytoplasmic transport proteins and in terms of the solubility of free heme in membrane bilayers.